The field of Pharmacogenomics seeks to delineate the genetic factors responsible for variations in drug response, both among individuals in one ethnic group and between population groups. To date, most effort in pharmacogenomics has focused on polymorphic variants in drug-metabolizing hepatic cytochrome P450 (CYP) enzymes. This proposal will address a previously unexplored potential basis for variation in drug metabolism, NADPH-dependent P450 oxidoreductase (POR). For microsomal cytochrome P450's POR is the only source of the electrons necessary to achieve catalysis. POR accepts electrons from NADPH and transfers them to the P450. Although the phenotype of POR knockout mice is an embryonic lethal, surprisingly we recently found that human POR missense mutations are responsible for a broad spectrum of human disease, ranging from a normal female phenotype with amenorrhea, to severely malformed infants with the Antley-Bixler syndrome (ABS). Other data suggest that POR mutations disrupt fetal drug metabolism, apparently rendering an otherwise benign drug (fluconazole) teratogenic. The frequency, severity and ethnic distribution of POR missense mutations, either those severe enough to cause disease or those that may be phenotypically silent, is unknown. To assess the potential role of POR sequence variants in variations in drug response, we propose the following four Specific Aims: Aim 1. Identify and characterize the spectrum of POR mutations causing identifiable human disease. Aim 2. Delineate the spectrum of POR sequence variants and polymorphisms in the normal human population. Aim 3. Assess the impact of POR missense mutants or variants on the eight most important hepatic P450 enzymes (CYP1A2, 2A6, 2C8, 2C9, 2C19, 2D6, 2E1, and 3A4). Aim 4. Determine whether sequence variants of POR and CYP enzymes that have minimal loss of activity individually can, in combination, lead to significant diminution in the activity of these enzymes. Successful completion of these aims will substantially increase knowledge about the genetic factors leading to variations in drug response.